The oxidation of hydroxybenzyl alcohol by means of air or pure oxygen in the presence of a catalyst based on a noble metal such as platinum or palladium has been reported..sup.1 The oxidation is generally carried out in the liquid phase in an aqueous, preferably alkaline, medium. In the case of ortho-hydroxybenzyl alcohol, the oxidation in an aqueous alkaline medium containing about 1 percent by weight of noble metal relative to the alcohol to be oxidized gives salicylaldehyde with yields which are between 70 and 80 percent, relative to the alcohol employed. However, it is found that this oxidation method needs extended reaction times, of up to 45 hours, to give a high degree of conversion to the aldehyde. The addition of boric acid or a borate in substantially equimolecular quantities in relation to the hydroxybenzyl alcohol to be oxidized, reportedly speeds up the oxidation without promoting the formation of secondary products..sup.2 However, this process using the boric acid or borate additives requires a reaction time from one to several hours depending upon the starting material employed. FNT .sup.1 Marchand et al., U.S. Pat. No. 3,321,526 (May 23, 1967). FNT .sup.2 Note 1, supra.
Another method of oxidizing the hydroxybenzyl alcohols to the hydroxybenzaldehydes comprises liquid phase oxidation of the corresponding hydroxybenzyl alcohol with molecular oxygen or of a gas containing molecular oxygen in an aqueous medium containing an alkaline agent, in the presence of a platinum or palladium catalyst and in the presence of a cocatalyst containing bismuth..sup.3 The use of the platinum/bismuth catalyst system reportedly gives a significant improvement both in respect of the rate of reaction and in respect of the yields of aldehyde; it also makes it possible to significantly reduce the amount of noble metal usually employed. FNT .sup.3 Ludec, U.S. Pat. No. 4,026,950 (May 31, 1977)
Hence, the prior art methods usually require a long reaction time with resulting low productivity or require the use of additives, e.g., bismuth compounds, boric acid or borate compounds in order to obtain shorter reaction times or better selectivity of the oxidation reaction. It would be desirable to prepare aldehydes from the corresponding alcohols in a more frugal and expeditious manner.